1. Field of Invention
This invention relates to a new and improved sensing head assembly for use as an on-line densitometer on multi-color printing presses.
More particularly, the invention relates to a sensing heat assembly which senses and derives output electric signals representative of the small spot reflectance of the ink colors (hence, density of the ink) used in a multi-color printing press. The assembly is designed for use on-line during normal printing operations of the multi-color press to provide continuous monitoring of the printing ink density. The output signals derived from the assembly can be employed to provide a visual read out to a press man who then can appropriately adjust the ink characteristics as it is applied to maintain some preset standard. Alternatively, the output signals can be applied directly to control ink application by an automatically controlled printing press.
2. Prior Art Problem
U.S. Pat. No. 3,756,725 -- issued Sept. 4, 1973 discloses an apparatus and method for determining the density of color reproduction in a multi-color printing press while on-line during printing operations. The on-line densitometer disclosed in U.S. Pat. No. 3,756,725 employs a pulsed, elongated flash lamp light source which illuminates a multiplicity of test patches and adjacent reference surfaces simultaneously, and the pulsed light source must be gated-on in synchronism with the movement of the color test patches and adjacent reference surfaces past an inspection zone. Such an arrangement possesses built-in complexity that necessitates the inclusion of complex and expensive synchronization circuitry for the pulsed flash lamp light source. To maintain this synchronizing circuitry in proper working order requires highly paid, skilled technicians and requires a relatively high initial cost for manufacture and installation of the system. In addition, the use of the pulsed flash lamp light source to derive the color density output signals, introduces undesired transients in the normal operation of the instrument which complicate its calibration and proper alignment. Further, the use of an elongated light source for illuminating a multiplicity of color bars extending across the width of the material being monitored, can introduce non-uniformities in illumination of the color bars as they are read out. To make available an improved on-line densitometer sensing head assembly which does not possess these undesirable characteristics, the present invention was devised.